This invention generally relates to electronic systems and in particular it relates to driver circuits with tail currents in discrete subranges.
FIG. 1 shows a prior art driver for optical network laser diodes. The circuitry 20 is typically integrated onto one chip. The xe2x80x9cbase driverxe2x80x9d blocks 22 and 24 convert positive emitter coupled logic (PECL) levels used up to this point to levels appropriate for driving the bases of the final differential pair devices 26 and 28. Resistor 30 is included to pad the dynamic resistance of the laser diode 32 up to a value matching the stripline transmission line impedance used on the circuit board. A dummy load 34 is provided to ensure the differential pair symmetry is undisturbed. Varying amounts of modulation current IMOD and laser bias current IBIAS are supplied by current sources 36 and 38. Generally these sources are actually enclosed in feedback paths to compensate for laser diode temperature and aging drift. The amount of modulation current needed may vary from a few milliamps for a typical vertical cavity surface emitting laser (VCSEL) to 100 milliamps or more for an edge emitting laser (EEL). The circuit of FIG. 1 also includes inductors 40 and 42, capacitor 44, and source voltage VCC.
The bipolar devices 26 and 28 in FIG. 1 will exhibit a marked dependence of transition frequency fT on emitter current density JE. FIG. 2 is a plot of a typical device characteristic. As future needs for bit rates are approaching the limits of available processes, it is necessary to optimize current density for amplifier bandwidth. However, if the modulation current can vary over an order of magnitude or more, it is clear that for some lasers the differential pair devices will be operating at current densities giving decidedly suboptimal bandwidths.
Another problem with the prior art device of FIG. 1 is that the current source 36 is formed by a transistor that must be sized to allow the total maximum modulation current to safely flow through it. This results in a very large device, with a large collector-to-substrate capacitance, which adds considerable parasitic capacitance on the differential tail node, the common emitter connection of transistors 26 and 28. When the base drivers 22 and 24 change the base voltages of transistors 26 and 28 in order to change the state of the laser or other output signal, the voltage at this tail node will fluctuate due to the nonlinear behavior of the bipolar transistor. This means that the large parasitic capacitance at that node must be charged with current, and the only place where this current can flow from is the output circuit. In practice, large glitches can be seen on the output current waveform flowing in the collectors of transistors 26 and 28. This is undesirable, as it distorts the ideally square output waveform and can cause errors in the communications system.
A driver circuit includes an array of differential pairs. Each one of the differential pairs is coupled to a corresponding tail current source. The total modulation current for the driver circuit is developed as the sum of the output current from the array of differential pairs. Each of the tail current sources generates a subrange of the total range of modulation current. The tail current in a given differential pair will then only vary over a small subrange of the total modulation current range, and the device size in each pair may be optimized to keep the emitter current density near the level that gives optimum bandwidth. This is equivalent to electrically increasing the emitter area of the composite differential pair as the total modulation current is increased, keeping current density approximately constant at its optimal level.